A) Field of the Invention
This invention relates to a sound field controlling apparatus used in a public-address system.
B) Description of the Related Art
A public-address system is necessary when a speaker and an audience are in the same room and the audience cannot hear sufficiently what the speaker says because the room is large to some extent.
FIG. 5 shows an example of a structure of a conventional public-address system. In the example shown in the drawing, a microphone 71 and a plurality of loudspeakers 800-80n are equipped in a hall or meeting room 70, and a voice picked up by the microphone 71 is reinforced so that the audience can hear the voice from the loudspeakers 800-80n. At this time, acoustic feedbacks with loop properties H0-Hn exist.
A voice signal obtained by the microphone 71 is amplified by a head amplifier 72, converted to a digital signal by an A/D converter 73 and input to a digital signal processor (DSP) 74. The DSP 74 executes functions such as equalizing, controlling a delay time given to an input signal, controlling a level of an input signal, etc. After passing through an equalizer 75, the input digital signal is distributed to a plurality (n+1) of output lines, each corresponding to the plurality of loudspeakers 800-80n. Thereafter, the distributed signals are respectively processed by equalizers 760-76n, delay time and level controllers 770-77n, each of which are dedicated to each one of output lines, and then output to the loudspeakers 800-80n via D/A converters 780-78n and power amplifiers 790-79n.
The equalizer 75 and the equalizers 760-76n compensate the loop property. The equalizer 75 controls the loop property (acoustic feedback property) that is common to all of the output lines, and each of the equalizers 760-76n that are equipped in correspondence to the output lines respectively controls a loop property to the microphone 71 from corresponding one of the loudspeakers 800-80n. Besides, the loudspeakers 800-80n can be omitted.
The delay time and level controllers 770-77n control delay times given to reinforced signals sounded form the loudspeakers 800-80n and control the volume levels of the reinforced signals. The delay times corresponding to distances from a position of the microphone 71 (a source position) are given to the reinforced signals sounded form the loudspeakers 800-80n so that the audience can hear a direct sound from the speaker and the sound from the loudspeakers 800-80n at the same timing, and the levels of the reinforced signals sounded form the loudspeakers 800-80n not to generate a howling by the acoustic feedback.
Further, in the publication of Japanese Laid-open Patent H09-247787, a sound field controlling apparatus for restraining a howling by optimizing a system structure automatically or manually in a public-address system having a plurality of microphones and a plurality of loudspeakers. The sound field controlling apparatus comprises means for measuring a transfer function between each microphone and each loudspeaker, calculates information such as howling margin and a frequency response necessary for system architecture for each combination of the microphone and the loudspeaker by using the measured transfer function. Thereafter, the calculated information is output to provide it to an operator or used for modifying a mixing setting and amplification rate automatically.
In the above-described conventional public-address system, a position of the microphone for picking up sound is fixed, and an input from the microphone of which position is fixed is sounded from one or plurality of loudspeakers after adjusting a delay time and loop property.
In this case, there is no problem if the microphone is at a predetermined position (addressing position). However, when the speaker moves with using a wireless microphone so that the position of the speaker changes to some extent, loop properties H0-Hn to the microphone changes largely, and it makes howling unstable and affects to a sound quality.